In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, some communications networks are enabled to provide dual connectivity. Details of dual connectivity for Release 12 (Rel-12) in the suite of Long Term Evolution (LTE) telecommunications systems are provided in 3GPP TR 36.842 which introduced the concept of Master eNB (MeNB) and Secondary eNB (SeNB), where eNB is short for evolved NodeB. Each eNB then represents a separate cell for the wireless device.
Dual connectivity can be defined as operation where a given wireless device consumes radio resources provided by at least two different network points (a MeNB and at least one SeNBs), where the MeNB and the at least one SeNB are operatively connected over a non-ideal backhaul, implying that the MeNB and the at least one SeNB may only exchange information with a latency too large for joint resource scheduling. Moreover the MeNB and the at least one SeNB are assumed to use individual (i.e., separate) frequency carriers.
The performance of the communications network can be assumed to depend on how much data each eNB (MeNB as well as SeNB) receive from the serving gateway (SGW) of the wireless device in the communications network to transmit to the wireless device. One challenge with dual connectivity is to schedule resources for a specific wireless device on the frequency carriers of the MeNB and the at least one SeNB without being able to synchronize or exchange information in real time.
In comparison, in a communications network based on single cells, the serving gateway (SGW) of the wireless device transmits all downlink traffic for the wireless device to one eNB, i.e., the serving eNB. The serving eNB then schedules the wireless device and transmits the traffic downlink to the wireless device. However, as disclosed above, with dual connectivity, there could be more than one eNB serving the wireless device. In this case, the serving gateway may need to decide how to steer the traffic to the wireless device to two or more eNBs.
Two examples of splitting transmission of a flow of data destined for the wireless device between the MeNB and one SeNB in a dual connectivity scenario will be summarized next.
According to a first example the complete flow of data is split at the SGW. Part of the data for the wireless device is sent to the MeNB and the rest to the SeNB. Each eNB then transmits its flow of data on its own frequency carrier to the wireless device. The wireless device thereby receives the flow of data on two frequency carriers and can rebuild the complete flow of data.
According to a second example the complete flow of data is sent to the MeNB representing the serving eNB of the wireless device. The MeNB performs the split of the flow of data, whereby the MeNB keeps a share of the data and forwards the rest of the data to the SeNB. Each eNB then transmits its flow of data on its own frequency carrier to the wireless device. The wireless device thereby receives the flow of data on two frequency carriers and can rebuild the complete flow of data.
Two mechanism for splitting the flow of data between the eNBs in a dual connectivity enabled communications network, and potential drawbacks related thereto, will be summarized next.
According to a first example, equal splitting is performed, whereby each eNB is provided the same amount of data in the flow of data for the wireless device. However, if one of the eNBs is overloaded, this eNB will not be able to process its share of the data as fast as the other eNB(s) and the wireless device may experience a large delay.
According to a second example, the splitting is based on throughput and feedback. Each eNB is first provided some part of the total amount of data. Then each eNB feeds back its current throughput to the SGW (or the serving eNB). Then the eNB with the highest throughput is iteratively and gradually provided more data to transmit to the wireless device. However, if the wireless device is physically moving when receiving the flow of data, its current throughput does not necessarily reflect at all its future throughput. Similarly, if the conditions in a cell served by one of the eNBs change quickly (such as many new wireless devices entering the cell) the load of this one eNB can change drastically. Moreover the feedback interfaces are comparatively slow so that even if the conditions change only slowly, the feedback may arrive so late at the SGW that the current throughput is outdated and possibly incorrect. This can lead to yoyo effects where a wireless device, that is located at a cell border and receives a large amount of data from an eNB having poor performance, and that reports back poor traffic throughput, in the mean time moves towards the cell center and once in the cell center has a small amount of data to be delivered to it.
US 2015/0215945 A1 discloses mechanisms for enhancing wireless device buffer state reporting and logical channel prioritization procedures to communicate and manage multiple schedulers from different base stations in a dual connectivity system. Buffer status values are determined based on an allocation rule and a determined amount of available data for transmission. The mechanisms in US 2015/0215945 A1 are disclosed to be implemented by the wireless device and hence leave open how to handle traffic steering from the network side.
Hence, there is still a need for an improved traffic steering in dual connectivity enabled communications networks.